OEPP/EPFO Bull. No 2 : 91-96 (1971)

Problems in the Formulation of Pesticides for ULV Application

by J.N. HOPE S.D.C. Pesticides Limited. London [United Kingdom]

ABSTRACT The various factors afEecting the production of satisfactory ULV formulations

of common pesticides are discussed. Both the selection of carrier oils and the incor- poration of active ingredients are examined and field workers are warned against possible misfortunes as the result of I( on the spot >> formulations. The importance of using machines capable of emitting the carefully controlled droplets necessary for satisfactory ULV spraying is stressed.

Introduction

As far as ULV spraying is concerned, I have already heard several cases where the technique has been abandoned as being unsatisfactory by people who have understood it imperfectly and have either used equipment incapable of pro- viding the precise droplet spectrum necessary or chemicals which were not formulated for this task. I think it important to remember that while certain extremely potent active ingredients, when used against highly susceptible pests, can provide satisfactory results when poorly formulated and applied through equipment which has a highly variable performance, this type of result should not mislead people into believing that such a haphazard approach will always bring success.

I have noted a tendency on behalf of some field workers to assume that if a particular pest is satisfactorily controlled at medium to high volume by the appli- cation of an emulsifiable concentrate diluted in water, that it is sufficient to take an oil which is not obviously phytotoxic and simply mix in the conventional emul- sifiable concentrate. While sometimes this procedure will be acceptable, there will be occasions when it may lead to serious problems. At this relatively early stage, when there is so much interest in a technique which offers so many apparent advantages, it would be a pity if workers abandoned it because of disappointments which could have been avoided.

I propose, without attempting to provide any specific answers, to try and define in general terms the problems faced by ULV pesticide formulators.

Carriers

While there would seem to be no valid reason why materials other than oil should not be used satisfactorily as carriers of pesticides €or ULV treatments, and there is no doubt that a large domain of interesting research is open in seeking ways of modifying water to reduce evaporation, experience to date suggests that in many cases pesticides for ULV work are best formulated in non-phytotoxic oils and that frequently the oil carrier can provide supplementary pesticidal effects.

91

Oils with the following specification :

light-medium ; -. denomination: agricultural spray oil for foliage treatment - medium or

- viscosity : 4 to 7" Engler at 20" C : never lower than 3" Engler at 30" C ;

- density : 0.83 to 0.93 : if possible less than 0.90 ;

- distillation: 50 % distillant between 325 and 340 maximum ;

- evaporation: less than 20 % in the Lallao test;

- index of sulphonation: (U.N.R.) 85 to 90 % minimum ;

-. aromatic content: (Cornelisson and Watermann) less than 10 to 12 % ;

- nature : either paraffinic or naphthenic ;

- acidity : less than 0.16 (in K20) ;

have proved to be generally useful as pesticidal carriers for foliage work; however, it cannot be assumed that in all cases they would be ideal.

The potential phytotoxicity of an oil carrier is one of the most obvious characteristics which demand attention. However, it should be noted that oils which are phytotoxic when sprayed to near run-off in large droplets, may not present the same hazard when atomised more finely and it might, for economic or other reasons, be unwise to abandon them.

Though the likelihood is small, it is possible that oils which have a demon- suably low phytotoxicity when applied alone (as when used in emulsion for scale insect control at higher volumes) may possibly combine with an apparently non- phytotoxic pesticide to produce a mixture which is itself hazardous.

The task itself may affect the choice of carrier oil. Oils which are apparently safe when used on the foliage of a particular crop at a particular stage, may be less certain if repeated applications become necessary and there is the likelihood of a build-up. The phytotoxicity can itself be a discreet phenomenon, with foliage apparently undamaged but crop levels ultimately depressed. This type of effect has been noticed in the past when using commercial diesel oil as a carrier for DDT on cotton.

The meteorological circumstances under which the oil based pesticide is designed to be used can have an obvious bearing on the choice of oil. Oils with a low vapur pressure may be useful and highly desirable when used on crops grown under glass in temperate climates but the rate of evaporation may be too high to obtain satisfactory deposits in the drier areas of the sub-tropics. I think that it can be generally stated that if the atomising system is chosen for the production of droplets whose size is controlled within fine limits, then ir is desirable that the droplets maintain their dimensions at least for sufficient time for them to deposit on the target, and low volatility would help to ensure this. There seems little point in producing droplets which will evaporate rapidly and therefore diminish in mass, with a consequent reduction in their terminal velocity. Having said this one could

92

envisage a hypothetical situation in which it would be desirable that at least some of the volume of liquid applied to a given area be emitted in droplets whose size would rapidly diminish and perhaps ensure better penetration of certain types of canopy.

Past work by CUILLE and GUYOT (1954) at IFAC and others using spray oils applied ULV to control sigatoka on bananas, suggests that oils which have been too highly refined tend to lose their useful fungicidal characteristics and often when using oils as the carrier for fungicides it may be useful to retain the fungicidal characteristics in the carrier. The variable content of oils from different sources may well affect both the insecticidal and fungicidal characteristics of the final formulation. These may also play a role in interactions between the oil and the pesticide, which could be undesirable, particularly when any length of storage is involved.

In the past, vegetable oils have been limited in their usefulness to pesticidal formulations, due to the speed with which they tend to oxidise. However, it may well be possible to modify those undesirable characteristics and obtain some benefit.

Active Ingredients

Solutions

Clearly, from all points of view, true solutions of the pesticide in the carrier are more satisfactory than suspensions. While in some cases, the pesticide may be brought into solution in the oil by merely leaving it in the presence of the oil, sometimes both heat and co-solvents are necessary to hasten the process or indeed to accomplish solution at all. The choice of co-solvents can seriously affect the phytotoxic potential of the final solution. The field worker may be tempted to believe that the mere mixture of an emulsifiable concentrate (in which the initial work of solution has been done for him) with a non-phytotoxic oil will provide a satisfactory formulation. However, the emulsifiers and other ingredients of the emulsifiable concentrate may precipitate or cause other difficulties.

Undoubtedly, the hardest task of the ULV pesticide formulator is the pre- paration of really satisfactory suspensions of insoluble pesticides, such as the more commonly employed fungicides (maneb, copper oxychloride, etc.) or insecticides (carbaryl). Ostensibly, the task is to reduce the pesticide to the finest dimension possible and then ensure its thorough incorporation into the oil. The task of reducing solids to fine powders requires a special understanding of milling techniques and the availability of high cost capital equipment. The particle size of a pesticide which might provide the most satisfactory water dispersable powder may not be identical with the optimum particle size for producing a e ready-for-use > oil based suspension. While the finer the particle size, the slower will be its initial deposition, some suspensions which are too finely ground tend to form very solid cakes at the bottop of containers when sedimentation is completed and these deposits are not always easy to bring back into suspension, so that each active ingredient has got to be examined independently from this point of view. For unless in formulation

93

you obtain Brownian movement, sedimentation is an inevitability and the best that the formulator can do is to try to provide a sediment which can very easily be brought into re-suspension. It is also often necessary to treat the particulate ingre- dient in such a way as to discourage the agglomeration of particles, since when this does take place, their future separation can be very difficult.

Given the optimum particle size, the incorporation of the powder into the oil frequently requires highly sophisticated milling equipment, which can usually only be found in a chemical factory. A problem in formulating oil suspensions, which may be less obvious to the uninitiated, is that certain pesticides tend to be oleophobic and when this is the case, either the oil or, more usually, the active ingredient has to be treated to overcome this problem.

From what has been said above, it must be clear that merely producing a finely ground water dispersable powder and attempting to mix it with apparently non-phytotoxic oil is unlikely to provide the type of formulation which will prove useful in a long-term crop protection program. I hope that two points might emerge from what I have said.

1. That although ready-for-use ULV products usually contain only a small percentage of active ingredient and a large quantity of relatively cheap oil carrier (and therefore the field worker might find it difficult to understand how the formu- lator justifies the price he is obliged to charge), the procedure necessary to produce a satisfactory and safe formulation is far more complicated than he would ever imagine.

2. While there is an obvious desire in some territories to do as much local formulating as possib!e,' the business of producing the ULV formulation can demand a high capital equipment facility and would rarely be justified in supplying the local demand for the finished product. Since particular formulations have specific tasks, they may require the use of different carrier oils and it is unlikely that these oils will be stocked in many countries where theoretically it would be desirable to formulate locally. If it were necessary to import both the pesticide ingredients and the carrier oil as special shipments, then the final product is likely to be a good deal more expensive than if the e ready-for-use. formulation were obtained else- where. Local production could not be justified on the grounds of the provision of additional tasks for labour, since formulation chemistry is an industry with a low labour requirement.

After 17 years experience in the formulation of ULV products, I frankly admit there are many problems which still await solution. Most of them are not within the domain of the chemist; for example, a great deal of further work is necessary to establish basic parameters. No one to my knowledge can tell me whether an even, dense deposit of a particulate insecticide applied in oil to a plant surface is likely to provide better control of a specific pest than an even (but much less dense) deposit of the same insecticide presented in larger particles. Our col- leagues working in the field of stored products insect control made some revealing discoveries within the last two years which have upset previous theories. They found that a given quantity of insecticide would provide better protection if sprayed thickly around only some of the cereal grains in the silo, rather than if the same dose of insecticide was sprayed uniformly over all the grains in the silo. There are a myriad such questions still to answer and when our knowledge is perfected then the formulator will be in a better position to provide more useful pesticides.

94